Trac #16183 was caused by TH conversion (in `Convert`) not properly
inserting parentheses around occurrences of explicit signatures where
appropriate, such as in applications, function types, and type family
equations. Solution: use `parenthesizeHsType sigPrec` in these
places. While I was in town, I also updated `nlHsFunTy` to do the
same thing.

(cherry picked from commit b1e569a5)